see but that seems like more of a mathematical proof, for all we know a quark could attract itself, without a positive or a negative charge (i don't believe such in the slightest), but i think if it's only mathematical we're grasping at straws when we say a neutron is 2 down and 1 up
i don't...
we have physical evidence for the composition of a proton because it decays from a neutron
however all experiments such as https://en.wikipedia.org/wiki/Deep_inelastic_scattering etc, always involve a proton and possibly a neutron, in fact you can't accelerate a neutron without a proton...